Field of the Invention
The present invention relates to an optical modulator element an optical modulation module including the optical modulator element, and a method for manufacturing optical modulator element.
Description of the Background Art
The recent data communications with high volumes of data traffic require optical fiber communication systems having increased capacity. Modulation schemes used in such systems include phase modulation and polarization-division multiplexing, in which multi-level signals are used to increase the channel capacity at a given band rate. Under these methods, Mach-Zehnder optical modulators are used to modulate signal light.
A light beam received by such a Mach-Zehnder optical modulator is split into two paths by an optical coupler, and then, is output when recombined by another optical coupler. The incident light beam is modulated with a phase shift that occurs due to refractive index variations caused by an electric filed applied to a medium in at least one of these paths.
Under the schemes such as the multilevel phase-shift keying using four or more phases, two Mach-Zehnder optical modulators are used to perform modulation. The incident light beam is split by an optical coupler, separately modulated with 90-degree phase shift by the respective modulators, and recombined by another optical coupler. Meanwhile, the polarization-division multiplexing requires different modulators for different directions of polarization (see, for example, Japanese Patent Application Laid-Open No. 2014-092713).
The conventional optical modulator elements in the polarization-division multiplexing scheme include modulators based on the electro-optic effect in crystalline materials such as lithium niobate (LiNbO3). With advances in miniaturization and cost reduction in view, optical modulator elements have been developed which is formed on a semiconductor substrate made of indium phosphide (InP) or silicon (see, for example, Japanese Patent Application Laid-Open No. 2010-185979 and Japanese Patent Application Laid-Open No. 2014-164243).
However, the following drawbacks have been constraints for miniaturization of semiconductor optical modulator elements and optical modulation modules including the modulator elements.
Firstly, the conventional configuration including two optical modulator disposed in parallel with each other fails to minimize the gap between these modulators. This configuration is subject to spatial constraints associated with the outer diameter of a collimator lens disposed on the output side of the individual optical modulator. With the gap being required by two collimator lenses adjoining each other, this configuration fails to increase the proximity of two modulators any further.
Secondly, the above-mentioned configuration requires that high-frequency wave signals propagate through each optical modulator with the same amount of delay. Thus, high-frequency wave transmission lines are routed in such a manner that the length of each transmission line becomes equal to that of the longest one of the transmission lines. Thus, a larger area is required for the high-frequency wave transmission lines.
Thirdly, the path of the incident light beam interferes with the paths provided by the high-frequency wave transmission lines in the module. Alternatively, the high-frequency wave transmission lines may bypass the optical paths and optical components. However, this configuration requires a larger area for the high-frequency wave transmission lines, failing to reduce the module size.